1. Field of the Invention
The present invention relates to a light source for optical communication. More particularly, the present invention relates to an optical modulator module including a distributed feedback laser.
2. Description of the Related Art
Optical communication has several advantages over standard electrical communication in that optical communication can simultaneously transmit more signals to a long-distance destination, as compared with normal transmission of electric signals. As a light source of the above-mentioned optical communication, a laser capable of outputting coherent light is widely used. In particular, a semiconductor laser is widely used as well, wherein the laser is fabricated by a semiconductor fabricating process and is thus miniaturized.
With regard to recent developments in optical communications, a wavelength division multiplexing (hereinafter, referred to as “WDM”) system has been used. According to this system, WDM simultaneously transmits/receives a plurality of optical signals having different wavelengths from each other and provides improved optical communication capacity and speed. The WDM optical communication uses a distributed feed laser as its light source. The laser is capable of generating an optical signal having a narrow oscillation linewidth or can be used with active optical devices capable of selectively outputting optical signals of a specific wavelength.
Wavelength filtering by using a diffraction grating is a popular filtering method, and both the distributed feedback laser and functional optical devices select a specific wavelength in a WDM optical communication network to provide filtering. In particular, the type of distributed feedback laser having a narrow oscillation spectrum required for increasing transmission distance in optical communication is well-suited for wavelength filtering by diffraction grating. There are also various types of diffraction grating structures that have been developed and disclosed in various technical publications.
With regard to semiconductor optical devices, it is noted that these devices sense the periodic change of a refractive index of an optical wave advancing toward the semiconductor optical device by wavelength filtering, and such devices reflect only a specific wavelength corresponding to a Bragg wavelength that is fed back to a gain region, thereby oscillating an optical signal having the specific wavelength.
However, one of the problems with a conventional light source is that there must be a separate modulation means for converting an electrical signal into a modulated optical signal. In addition, there are problems associated with using the conventional light source, such as an increase in the number of manufacturing processes required, a larger percentage of defects, both of which increase the unit costs of production.
In order to solve at least some of the above-mentioned problems, there have been attempts in the art to provide a monolithic integrated semiconductor photo device that comprises a distributed feedback laser and an electro-absorptive modulator that are integrated on a single substrate.
FIG. 1 is a illustrates a typical construction of the conventional electro-absorptive optical modulator module. Referring to FIG. 1, the conventional electro-absorptive optical modulator module includes an optical modulator and an optical detector 150. The optical modulator includes a distributed feedback laser 110 and an electro-absorptive modulator 120 which are integrated in the module. The optical detector 150 is arranged opposite to one end of the optical modulator.
Also, as shown in FIG. 1, the optical modulator includes a substrate 101, a lower clad 104 formed on the substrate 101, a first and a second active layer 105a and 105b formed on the lower clad 104 by a selective region growing method or a butt joint method, so that the active layers 105a and 105b may have different bandgaps from each other, and an upper clad 106 formed on the first and second active layers 105a and 105b. 
The lower clad 104 includes a buffer layer (not shown) formed on the substrate 101, and an n-InP layer (not shown) formed on the buffer layer.
The first active layer 105a comprises a gain medium of the distributed feedback laser 110 and generates light having a predetermined wavelength. The distributed feedback laser 110 forms gratings having a predetermined period on the lower clad 104. Consequently, the light generated from the first active layer 105a is oscillated to be output to the electro-absorptive modulator 120.
The second active layer 105b is a gain medium of the electro-absorptive modulator 120 and progresses the light output from the first active layer 105a inside the second active layer 105b while the absorbance of the light is changed according to an applied voltage, thereby modulating the intensity of the light.
The upper clad 106 may include a p-InP layer (not shown) formed on the first and second active layers 105a and 105b, a contact layer (not shown) formed on the p-InP layer, etc.
The distributed feedback laser 110 and the electro-absorptive modulator 120 include the first and the second active layer 105a and 105b, respectively, which are continuously grown on the substrate 101. The electro-absorptive modulator 120 outputs light received from the distributed feedback laser 110.
The optical modulator shown in FIG. 1 also has a high-reflection layer 130 that is coated on one end close to the distributed feedback laser 110, and an anti-reflection layer 140 that is coated on the other end close to the electro-absorptive modulator 120. These coatings permit the high-reflection layer 130 to reflect light oscillated from the distributed feedback laser 110 to the electro-absorptive modulator 120.
The optical detector 150 can be constructed by providing a waveguide-type photodiode and arranging the waveguide-type photodiode opposite one end of the optical modulator on which the high-refection layer 130 is coated. With such a configuration it is the intensity of light output from the anti-reflection layer 140 of the optical modulator can be monitored according to an analysis of the intensity of partial light transmitted through the high-refection layer 130.
However, according to the conventional electro-absorptive optical modulator module such as the type shown in FIG. 1, the optical modulator and the optical detector are formed on separate substrates respectively, in which the optical modulator includes the distributed feedback laser and the electro-absorptive modulator and the optical detector function to monitor intensity of light oscillated from the distributed feedback laser. Accordingly, the optical axes must be necessarily aligned after the respective devices have been fabricated.
Therefore, as it is necessary to perform an optical-axis aligning process with the conventional electro-absorptive optical modulator module, there is a significant loss of process time and productivity. Furthermore, after the electro-absorptive optical modulator module has been fabricated, the optical axes may be out of alignment with each other (owing to various reasons), so that it is at times not possible to monitor the exact intensity of an optical signal.